The present invention concerns a nuclear fuel rod for a nuclear reactor of the boiling water or pressurized water type, comprising a cladding tube that defines a closed inner space and which is manufactured from at least one of the materials in the group zirconium and a zirconium-based alloy, a pile of nuclear fuel pellets arranged in the inner space in the cladding tube so that the nuclear fuel pellets fill part of the inner space, and a fill gas arranged in the closed inner space to fill the rest of the inner space, whereby the fill gas contains a proportion of helium and a proportion of carbon monoxide. The invention also concerns a nuclear fuel assembly comprising such a nuclear fuel rod. Furthermore the invention concerns a method for manufacturing a nuclear fuel rod for a nuclear reactor of the boiling water or pressurized water type.
A nuclear plant comprises a reactor core that is formed from a number of fuel assemblies. Each fuel assembly comprises a plurality of elongated, parallel nuclear fuel rods and a plurality of spacers that are axially distributed along, and associated with, the nuclear fuel rods. Each nuclear fuel rod comprises a cladding tube and nuclear fuel that is enclosed in the cladding tube. The nuclear fuel usually comprises uranium that is compressed into fuel pellets arranged on top of one another in a pile in the cladding tube. When the nuclear plant is in operation the reactor core is cooled by means of a cooling medium that is pumped upwards through the reactor core.
The components in nuclear plants are often subjected to attacks caused by hydrogenation and oxidation. It is known to provide such exposed components with a surface coating to protect the components. The nuclear fuel rods' cladding tubes are an example of such components. The attack of a cladding tube means that a defect extending through the whole thickness of the cladding tube occurs in the worst case, whereby the radioactive nuclear fuel and its fission products that are inside the cladding tube can leak out into the reactors cooling water. As regards defects to the cladding tubes, one differentiates between primary and secondary defects.
A primary defect occurs by attack on the outer surface of the cladding tube and is especially caused by abrasion by foreign objects. A minor abrasive defect does not normally give rise to any evident destruction and washing out of the nuclear fuel rod's uranium pellets. A primary defect can however extend through the whole thickness of the cladding tube. Such a primary defect means that water, steam, or a combination of these, flows into the cladding tube into a space between the nuclear fuel and the inner surface of the cladding tube.
When a primary defect has developed there is a communication path between the cladding tube's inner space and the reactor's cooling water. Water and steam will therefore force their way into the nuclear fuel rod until the nuclear fuel rod's internal pressure Pi is the same as the reactor's system pressure Psys. During this course of events the inside of the cladding tube and the uranium pellets oxidize during the release of hydrogen from the water molecules. This in turn results in that one can obtain an environment with a very high partial pressure of hydrogen, ppH2 so called “oxygen starvation” or “steam starvation” at a distance from the primary defect. In such an environment the inside of the cladding tends to absorb hydrogen very quickly, so called hydrogenation. The hydrogenation can lead to a very high hydrogen concentration locally in the cladding, which is called secondary degrading and which in turn strongly deteriorates the cladding's mechanical properties. The cladding becomes very brittle and this can give rise to the inducement of cracks, crack growth and the development of a secondary fuel defect due to self-induced stresses or due to outer loading. A secondary defect often takes the form of long cracks or transverse fracture, which means that this is a serious form of defect.
If a defect is such that water flows into the nuclear fuel rod the water will be vaporized and water molecules will dissociate, whereby the cladding tube's inner surface oxidizes and free hydrogen is formed in the inner space. The free hydrogen will be absorbed by the zirconium-based cladding tube, whereby the above-mentioned embrittlement occurs. Absorption takes place particularly on the surfaces where an oxide layer has not yet formed. This process is relatively fast. The oxidation process is initiated immediately after a primary defect has occurred. This quickly results in the occurrence of an environment with very high partial pressure of hydrogen at a distance from the primary defect, as a consequence of this, the hours and days following the occurrence of a primary defect are of great importance for the possibilities of influencing the secondary hydrogenation and thereby the risk of a secondary defect occurring. A problem in this context is that hydrogen gas diffuses faster in the helium gas that is normally found in the inner space than water molecules. The hydrogen gas will therefore reach the free surfaces faster than the water molecules that could otherwise react with the surface during the formation of a protective oxide layer on the zirconium surface.
U.S. Pat. No. 4,609,524 discloses a closed tube for a nuclear reactor. The tube is intended to contain nuclear fuel and/or a neutron absorber. The fuel and/or the absorber are enclosed in the tube together with a fill gas consisting of He and an additional gas containing one of the gases O2, CO and CO2. The purpose of the additional gas is to provide a thin oxide layer on the inner surface of the cladding tube. The oxide layer is thought to reduce the permeability of triterium (an isotope of hydrogen) through the tube under normal operation. According to that which is stated in this document triterium is released during irradiation in the nuclear reactor. The document does not discuss the problems that occur in connection with a defect to the fuel rods. The purpose of the additional gas is therefore not to prevent hydrogenation and secondary degrading. The amount of the additional gas in the fill gas amounts to 2-3 volume percent of the amount of helium. The amount of additional gas in relation to the amount of fill gas is therefore less than 3 volume percent.